Efforts are currently underway for the preparation of [11C]MDL 100,907 [(R)-(+)-a-(2,3-dimethoxy-phenyl)-1-[2-(4-fluorophenylethyl)]-4- piperidinemethanol)] a high affinity antagonist for the 5HT2a serotonin receptor. The 3 C-11 methoxy derivative will be prepared. The synthesis of the parent racemic compound has been achieved using literature procedures. The compound has been resolved by separation of the diastereomeric esters of (S)-(+)-a-methoxyphenyl acetic acid using normal phase chromatography. The radiosynthesis of both the 2 and 3 methoxy derivatives has been described by others. Although both compounds show metabolites in the plasma, only the 3-[11C]methoxy derivative appears to be free from interfering metabolites. We are currently investigating other novel serotonin subtype receptor ligands for possible use as PET radiotracers.